Rats subjected to total sleep deprivation (TSD) by the disk-over-water method were provided with a continuously available operant by which they could increase ambient temperature (T(amb)). TSD rats progressively increased operant responses for heat to 700% of baseline levels. During the last quarter of sleep deprivation, they maintained mean T(amb) at 9 degrees C above baseline and held T(amb) over 40 degrees C for 35% of the day. In contrast, yoked control rats (TSC) did not change mean T(amb). Although both TSD and TSC rats showed a progressive decline in intraperitoneal temperature (T(ip)), TSD rats maintained an elevated T(amb) even during periods when T(ip) and brain temperatures (T(br)) were above baseline levels. Thus these results confirm and extend earlier findings that rats subjected to TSD show an increase in temperature set point (T(set)). The earlier studies, which utilized short daily trials in a thermal gradient, demonstrated an increase in T(set) early in the deprivation period. The present study, which obtained more extensive data on thermal preference at a range of body temperatures demonstrated an increasing T(amb) for almost all T(ip) and T(br) values, suggesting a progressive increase in T(set) over the course of sleep deprivation. Surprisingly, survival time was shorter than in previous TSD studies. Reduced survival could not be attributed to differences in T(br), T(ip), energy expenditure, or sleep loss from previous studies.